In commercial paint lines, paint is often applied with a hand held spray gun. For small jobs, the paint may be supplied from a suction feed paint cup attached to the gun. For larger jobs, the paint typically is supplied from a remote pressure feed pot or reservoir connected through a hose to the gun. The pressure feed paint pot typically is a container which holds a volume of paint and is pressurized with compressed air. Or, for large operations, a pump may be used to supply paint under pressure from a vented container. When the paint container is pressurized, it is made from metal to withstand the applied pressure without risk of bursting. A paint feed tube generally is mounted on the lid of both a suction feed container and a pressure feed container so that removal of the lid provides access to the interior of the container for cleaning or refilling. However, there is a tendency for paint to drip from the paint feed tube when the lid is removed from the containers. Further, air tends to enter the paint feed tube when it is lifted from paint in the container. A resulting bubble may flow to the spray gun which can adversely affect the quality of the applied finish.
To increase coating transfer efficiency in commercial applications, the atomized coating material frequently is electrostatically charged relative to workpieces being sprayed. The electrostatically charged atomized paint droplets are drawn from the gun to the workpiece by the electrostatic field, thereby producing a more complete coating and reducing the amount of coating material which is dispersed into the atmosphere. This reduces both material costs and potential environmental problems. A very high voltage is used to charge the atomized paint, typically between 60 and 100 or more kilovolts. Because of the high voltage, care must be taken to protect the equipment operator from electrical shock. When an electrically non-conductive paint is being applied, the shock risk is minimum since only the paint is charged as it is atomized at the gun nozzle.
In recent years, there has been increased use of water borne paints which release less potentially hazardous solvents into the environment. Water vapor is the primary solvent released into the atmosphere as water borne paints dry. However, water borne paints are electrically conductive and therefore difficult to apply with an electrostatic spray gun. When the paint is exposed to a high voltage at the gun, the entire volume of paint from the gun to the supply reservoir and any electrically conductive components in contact with the paint become charged. If the supply reservoir or any other location in the paint supply system comes into contact with an electrical ground, the electrostatic voltage will be discharged. Consequently, it is necessary either to provide a voltage block in the paint feed system or to electrically isolate the entire paint supply system from ground. In commercial operations, the conductive paint often is supplied to an electrostatic applicator from a paint pot placed on an electrically insulated stand. The pot and stand are then enclosed in a grounded protective cage. In order to open a door to the cage, a switch is actuated to turn off the high voltage and to ground the paint pot. Such an arrangement has a number of disadvantages. It is difficult and time consuming to fill the paint pot. Also, the cage requires a significant amount of floor space which may be at a premium and relatively long paint supply lines are needed. If the paint is supplied from a pressurized pot, an agitator may be required to maintain the paint suspension and the paint in the supply lines is not easily agitated. Recirculation through the paint line is not practical when a pressure pot is used. Further, the use of agitators with water borne paint has sometimes caused a foaming problem in the paint. Because of their high costs and lack of portability, existing voltage block and paint supply isolation systems have not been suitable for smaller businesses which, for example, may apply on the order of 30 to 50 gallons (114 to 189 liters) of paint per month. Consequently, smaller businesses often did not have the option of using water borne paints in an electrostatic application system.